The present invention relates to furnaces and, more particularly, to a method of controlling a two-stage, gas-fired furnace.
In order to obtain increased efficiencies, it has become common practice to provide two stages of operation in a furnace, the first stage operating the furnace on low heat and the second stage operating the furnace on high heat. Generally, the low heat level of operation will be used where possible to thereby obtain increased operating efficiencies. When higher heating inputs are required, as, for example, when the actual temperature does not reach the set temperature within a predetermined time period of operation in a low heat operating condition. It is also common to initially operate in a high heat mode in order that proper burner light off is achieved and that the heat exchangers are quickly warmed up.
In changing between low and high heat modes of operation, there are three parameters which must be changed. First, the inducer motor speed must be set for either low or high speed so as to provide the appropriate amount of combustion air. Secondly, the gas valve must be placed in either the low or high fuel flow position to properly match the amount of combustion air from the inducer. This is commonly accomplished by either opening or closing the high fire solenoid valve. Finally, the blower must be operated at either high or low speed to accommodate the amount of heat that is being generated.
It is possible that a high fire solenoid will be closed to provide the high flow rate of gas when the furnace is operating in a low fire mode. This may occur, for example, when the system has been operating initially in a high fire mode and then switches to a low fire mode but with the high fire solenoid tending to stick in the closed position. If this occurs, poor combustion will result, which is undesirable for reasons of efficiency and safety. A system will normally respond to such a condition by sensing the resulting higher temperatures and responsively turning off the gas and the inducer, and then after an appropriate reset time period, the cycle is commenced again. If the valve remains in the stuck position, the system will continue to operate in shortened cycles and the heat output will not be adequate. Further, the system will continue to experience poor combustion on a repeated basis.
It is therefore an object of the present invention to provide an improved control system for a two-stage furnace.
Another object of the present invention is the provision in a two-stage furnace for the prevention of poor combustion conditions.
Yet another object of the present invention is the provision in a two-stage furnace for reducing the repeated occurrences of a particular fault condition.
Still another object of the present invention is the provision in a two-stage furnace for accommodating the condition of having the high fire solenoid on during periods when the system should be operating in a low fire mode.
Still another object of the present invention is the provision for a two-stage furnace control system which is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.